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The role of ethylene in regulating growth in tomato (Lycopersicon esculentum Mill.) during compaction stress was examined using wild-type (cv Ailsa Craig) and transgenic (ACO1 AS ) genotypes; the latter has a reduced capacity to produce ethylene. Ethephon or silver ions were applied to increase ethylene production or block its action. Shoot growth in both genotypes was comparable in uncompacted (1.1 g cm ؊3 ) and uniformly compacted soil (1.5 g cm ؊3 ). However, a 1.1/1.5-g cm ؊3 split-pot treatment invoked marked genotypic differences: growth was reduced in cv Ailsa Craig but was comparable to uncompacted control plants in ACO1 AS . As xylem sap abscisic acid levels were similar, abscisic acid was not responsible for inhibiting growth in cv Ailsa Craig. These genotypic differences in growth were accompanied by increased ethylene evolution in cv Ailsa Craig, suggesting that the ability of ACO1 AS to maintain growth in the split-pot treatment reflected its lower ethylene levels, a view supported by the observation that excising the roots in the compacted compartment reduced ethylene evolution and restored shoot growth in cv Ailsa Craig. Treatment with silver restored shoot growth in cv Ailsa Craig, whereas treatment with ethephon reduced growth in ACO1 AS . Thus, ethylene apparently has a key role in determining growth when tomato plants encounter differential soil compaction.Plants grown in compacted soil frequently exhibit reductions in root and shoot growth and stomatal conductance in the absence of significant effects on foliar water status

Figure 2. Time courses of (a) stomatal conductance and (b) leaf water potential for three genotypes of tomato, Ailsa Craig, ACO1 AS and notabilis. Plants were grown in a split-pot system in which both compartments contained either uncompacted (1·1 g cm -3 ) or compacted soil (1·5 g cm -3 ). A 1·1/1·5 g cm -3 split-pot treatment was also used. Both compartments of the 1·1/1·5 g cm -3 split-pot treatment were either supplied with water (Split-pot); alternatively, a solution containing 100 nm ABA was applied to the 1·5 g cm -3 compartment and water to the 1·1 g cm -3 compartment (Split + ABA). Double standard errors of the mean are shown.

in the compacted compartment were excised. These results clearly demonstrate the involvement of a root-sourced signal in mediating responses to compacted soil; the role of ABA in providing this signal and future applications of the compaction procedures reported here are discussed. Key-words:Hordeum vulgare (barley); compacted soil; abscisic acid (ABA); root to shoot communication. INTRODUCTIONSoil compaction adversely affects crop growth and yield in many parts of the world (Voorhees 1991). External compression increases the bulk density and shear strength of the soil, restricting the growth of roots and thereby limiting their ability to exploit available water and nutrients. Moreover, the associated decrease in pore space volume reduces permeability and the diffusivity of gases and may result in the formation of anaerobic conditions (Greenland 1977). The stress imposed by soil compaction may therefore involve two key elements, impeded rooting conditions and an anaerobic rooting environment.Various methods have been used to examine the responses induced when roots encounter compacted soil. Goss (1977) used ballotini of varying size supplied with aerated nutrient solution, while others have employed wire mesh with differing grid sizes (Scholefield & Hall 1985), waxy substrates of varying strength (Taylor & Gardener 1960) or, more recently, pressurized columns of sand (Young et al. 1997). In contrast, Castillo et al. (1982) and Andrade, Wolfe & Ferres (1993) compacted soil under field conditions to create a range of compaction levels. Bengough, Mackenzie & Diggle (1992) and Mulholland et al. (1996a) used soil columns which differed in bulk density but were uniform in moisture content, whilst Masle (1998) controlled resistance to root penetration by altering the water content of compacted soil. These and other studies show that root and shoot growth are generally reduced whenever compacted soil is encountered.Wilson, Robards & Goss (1977) demonstrated that roots ABSTRACTNovel techniques were devised to explore the mechanisms mediating the adverse effects of compacted soil on plants. These included growing plants in: (i) profiles containing horizons differing in their degree of compaction and; (ii) split-pots in which the roots were divided between compartments containing moderately (1·4 g cm -3 ) and severely compacted (1·7 g cm -3 ) soil. Wild-type and ABA-deficient genotypes of barley were used to examine the role of abscisic acid (ABA) as a root-to-shoot signal. Shoot dry weight and leaf area were reduced and root : shoot ratio was increased relative to 1·4 g cm -3 control plants whenever plants of both genotypes encountered severely compacted horizons. In bartey cultivar Steptoe, stomatal conductance decreased within 4 d of the first roots encountering 1·7 g cm -3 soil and increased over a similar period when roots penetrated from 1·7 g cm -3 into 1·4 g cm -3 soil. (Hartung & Davies 1991;Tardieu et al. 1991Tardieu et al. , 1992. It is well established that ABA accumulates in roots exposed to compacted soil (T...

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Isogenic wild‐type (Ailsa Craig) and abscisic acid (ABA)‐deficient mutant (flacca) genotypes of tomato were used to examine the role of root‐sourced ABA in mediating growth and stomatal responses to compaction. Plants were grown in uniform soil columns providing low to moderate bulk densities (1.1–1.5 g cm−3), or in a split‐pot system, which allowed the roots to divide between soils of the same or differing bulk density (1.1/1.5 g cm−3). Root and shoot growth and leaf expansion were reduced when plants were grown in compacted soil (1.5 g cm−3) but leaf water status was not altered. However, stomatal conductance was affected, suggesting that non‐hydraulic signal(s) transported in the transpiration stream were responsible for the observed effects. Xylem sap and foliar ABA concentrations increased with bulk density for 10 and 15 days after emergence (DAE), respectively, but were thereafter poorly correlated with the observed growth responses. Growth was reduced to a similar extent in both genotypes in compacted soil (1.5 g cm−3), suggesting that ABA is not centrally involved in mediating growth in this severely limiting ‘critical’ compaction stress treatment. Growth performance in the 1.1/1.5 g cm−3 split‐pot treatment of Ailsa Craig was intermediate between the uniform 1.1 and 1.5 g cm−3 treatments, whereas stomatal conductance was comparable to the compacted 1.5 g cm−3 treatment. In contrast, shoot dry weight and leaf area in the split‐pot treatment of flacca were similar to the 1.5 g cm−3 treatment, but stomatal conductance was comparable to uncompacted control plants. These results suggest a role for root‐sourced ABA in regulating growth and stomatal conductance during ‘sub‐critical’ compaction stress, when genotypic differences in response are apparent. The observed genotypic differences are comparable to those previously reported for barley, but occurred at a much lower bulk density, reflecting the greater sensitivity of tomato to compaction. By alleviating the severe growth reductions induced when the entire root system encounters compacted soil, the split‐pot approach has important applications for studies of the role of root‐sourced signals in compaction‐sensitive species such as tomato.

When plants encounter compacted soil, stomatal closure occurs and shoot growth slows. These responses occur in the absence of detectable changes in foliar water status. The use of genotypes with a reduced capacity to synthesize either ABA or ethylene has provided convincing evidence that ABA is responsible for providing the signal that regulates stomatal aperture, whereas increased ethylene production leads to an inhibition of shoot growth. Compaction results in an elevated export of ABA from the roots while enhanced ethylene synthesis is associated with increased expression of ACC oxidase in the aerial parts of the plant. Future work will explore the mechanisms responsible for regulating these events and the contribution of anaerobiosis to the stresses experienced by roots growing under compacted conditions.

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